1. Field of the Invention
The present invention relates to a layered board that is constructed by stacking a plurality of layers. The present invention particularly relates to a layered board applicable to equipment of LSIs (large scale integrations; generally, electronic devices), such as central processing units (CPUs) and memories, and an apparatus incorporating such a layered board.
2. Description of the Related Background Art
High-speed processing, small size, and light weight are all required in information communication terminals, such as cellular phones and personal digital assistants (PDAs). It can be, however, pointed out that adverse influences of wiring delays increase in electronic circuit boards as the processing rate goes up. The simplest method for solving such a disadvantage is to reduce the length of electric wires in chips or between chips to be as short as possible. This method can also achieve compact circuit boards.
Another problem, however, arises as the processing speed increases. This is the problem of electromagnetic interference (EMI) whose meaning is as follows. When electronic devices are disposed close to each other, the density of electrical wires increases even though the length of the wires decreases. As a result, when high-speed signals flow though signal wires arranged close to each other, electromagnetically-induced electromagnetic waves interfere with each other, and generate noises, thereby causing erroneous transmission of signals. Particularly, in mobile terminals, there occur more cases where the terminal is driven with larger current as its driving voltage decreases. Accordingly, the influence of the EMI is more serious.
As a method for preventing the EMI problem, there can be considered a method of enhancing electromagnetic compatibility (EMC) in each layer by layering ceramics boards, for example. This measure, however, has disadvantages in cost and yield. In addition, it cannot achieve an inherently EMI-free apparatus.
Further, when an electronic device is to be newly added on the basis of an existing wiring pattern, there is a fear that present circuits be adversely influenced by the added electronic device. In other words, it is sometimes desired to prevent adverse influences of the EMI caused by a specific device in the circuit.
From the above-discussed standpoint of view, there have been proposed methods of using optical wiring or interconnection which are inherently electromagnetic-induction-free, and can achieve EMI-free apparatuses.
U.S. Pat. No. 5,835,646 discloses the following active optical sheet, or active optical circuit board, for example. In this active optical circuit sheet, or active optical circuit board, an electro optical switch or optical modulator is driven with a voltage (SIGin) from an electronic device, the electrical signal (SIGin) is converted to an optical signal, transmitted and then converted to an electrical signal (SIGout) at an optical receiver element, and an electrical connection is formed between an optical wiring board and the electronic device for transmission of signals to another or the same electronic device, separating the electrical wiring at the electronic device end and the optical wiring at the optical wiring board end, or alternatively, SIGin and SIGout electrode pads are provided on the side of the optical wiring board on which the optical device is mounted or on the opposite side, for connection with the electronic device.
On the other hand, European Unexamined Patent Publication No. EP1219994A discloses a semiconductor device in which a plurality of electrical wiring layer and optical wiring layers are stacked, the electrical wiring layers is constructed based on a conventional scheme, and the optical wiring layer is constructed as a two-dimensional (sheet-shaped) optical waveguide layer.
Further, Japanese Patent Laid-Open No. 5(1993)-218384 discloses a semiconductor integrated circuit as illustrated in FIG. 8. In semiconductor integrated circuit of FIG. 8, converting devices (a light emitting device 805, and a light receiving device 810) for performing conversion between an electrical signal and an optical signal, and an optical waveguide 810 are integrally arranged in the semiconductor integrated circuit, and there is provided a unit for performing signal transmission in the integrated circuit through the converting devices and the optical waveguide. In FIG. 8, there are further arranged a silicon semiconductor substrate 801, a substrate 802 of a silicon oxide layer, a silicon semiconductor logical operation integrated circuit 804, a metal wiring layer 806, a grating 809, and an insulating layer 811.